Mike is right on,...
Watershields, although practical for most street use, are restrictive and do cost power and its both measurable & noticeable. Even the jetting is different between open air cleaners and the watershield versions.
Its just another compromise people make for practicality vs max performance so one makes choices.
I would say that if one decides to use an open aircleaner like Mike's or the similar ITG foam ones, one must learn to "bag" the aircleaners before washing the car and when parked outside if it rains. This keeps water out of the float chambers (a maddening experience) and prevents the engine from getting hydrolocked; something our 914/6 brethern know about.
K&N used to make a very tall version of these aircleaner elements that was a real tight fit, but really worked well. I no longer see them on their website so perhaps these have been discontinued.